Move BuildPlatform to dub.platform and improve docs.

Various small fixes ...

- fix computeBuildID for --compiler=/my/path;
- dub compiled with ldc tried to invoke ldc as default binary instead of ldc2;
- dmd: Don't generate anything;
- gdc: Do not link, only generate an object;

Fix ARM FP version identifiers.

Implemented basic build platform detection and extended the possibilities for platform specific fields in package.json.